U.S. patent application Ser. No. 455,161, filed Dec. 22, 1989, now U.S. Pat. No. 5,040,190 entitled "Analog Data Station Terminal" by M. Smith et al, assigned to the assignee of the present application and the disclosure of which is herein incorporated, describes an analog/digital interface device through which the digital signalling capability of a digital carrier telephone communication system can be extended all the way to the customer site, thereby eliminating a number of impairments normally encountered on local loops, including the need for equalization of analog lines. As described in the Smith et al application, such an analog/digital interface device, termed an analog data station terminal, has an analog channel port which interfaces directly with an analog channel from the customer, on the one hand, and a digital port which interfaces with a 56 Kb secondary channel mode, unloaded four wire digital pair from a central office (digital) channel unit data port, on the other hand.
The central office channel unit (OCU) data port is essentially a 64 Kb clear channel digital-to-digital interface commonly employed in digital communications systems to transport digital data between stations, via a prescribed digital carrier (e.g. a T1 or 1.544 Mb/s channel). As such, it is not capable of accepting analog tone signals conventionally employed by a telephone test facility to conduct diagnostic testing of the channel. Thus, although the analog data station terminal solves the problem of analog line conditioning, and thereby enables the telephone company to extend digital communications all the way to the customer premise, there still remains the problem of interfacing an analog tone-based test facility with signal format-incompatible digital signalling units. Also, a clear channel unit is not capable of performing zero suppression.
An additional shortcoming of a conventional OCU data port is the unwanted perturbation of an analog channel that results from converting a digital code byte used to execute prescribed control functions into analog format. Specifically, such code bytes are intended to be decoded by another OCU data port rather than being converted into an analog voltage for application to an analog channel. However, if the office channel unit is also ported to an analog/digital interface, such as the above-mentioned analog data station terminal, the analog-to-digital conversion of the control function codes will produce analog voltage levels which are substantially different from signal levels expected to be seen by downstream analog equipment, resulting in `glitch` on the analog channel.